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distance with type J thermocouple wire

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2571

Industrial
Aug 11, 2006
85
Good morning,
What is the maximum distance I can run type J thermocouple wire in a enviroment with an average ceiling temp of 140 degrees? Our plan is to reroute a temp controller to a dry off oven to remote locatation to better control oven temp during times the oven is not required to be on maximum temp settings. With out having to travel to the back of a department to adjust. We have Honeywell digital controllers DC3001.

thank you
2571
 
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We run type J, K and T thermoccouples for several hundred meters across a large industrial plant from the sensing position back to the DCS. The interconnection is made using compensating cable or extender cable which is compatible with the thermocouple material and reduces the likelihood of erroneous readings caused by joints or terminations on the cable run.


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Sometimes I only open my mouth to swap feet...
 
Much better would be some remote display. Can you talk to those DC3001 over RS485? You could then convert to Ethernet and read the temp from anywhere without risking noise pickup problems. Furthermore it would keep the controller near what it is controlling!

Keith Cress
Flamin Systems, Inc.-
 
Much better

Better? I'm unconvinced! A properly installed thermocouple feeding into a decent quality instrument is fairly immune to external pickup. Power plants are among the electromagnetically noisest places out there except arc furnaces and smelters and we don't often get problems unless we use crappy components, cheap instruments, or don't pay attention to the installation.

Some temperature indicators have a repeater output scaled as 0-10V or 4-20mA to drive an external indicator. If you have a mA output you might consider using that. If it is a voltage output, you're better off either moving the instrument, installing a converter to either digitise the signal into a data stream for RS-485 or ethernet, or else convert it to a mA signal.


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Sometimes I only open my mouth to swap feet...
 
Scotty it's really better to move the controller far away?!? Pishshaw! I say.[infinity]

Seems, to me, that unless you have a fully DCS facility or a big sterile power-plant that normally appears deserted, it is better to have a controller near the machine so workers can at least occasionally note the temperature/etc.

Keith Cress
Flamin Systems, Inc.-
 
Sorry, I thought he was moving the controller to a location where the operators were more likely to get to it and make the adjustment!
 
Ah.. you might be right. I just thought the placement should be considered further especially with respect to removing indication from the area. I agree there is no big problem with long T/Cs (usually).

Keith Cress
Flamin Systems, Inc.-
 
I've run 2-300 yards of TC extension wire, like others have, and had good results.

The spec sheet for that old (pre-1994) UDC 3001 states,
"Common Mode Rejection: Ac (50 or 60Hz): 120 db (with maximum source impedance of 100 ohms ) or ±1LSB whichever is greater.

That would infer that extension wire resistances in excess of 100 ohms would reduce the CMRR. Ungrounding the junction would do a lot to avoid common mode in the first place.

20g J is about 1/3 ohm per double foot, so it does add up.

Dan
 
I'm really amazed a company is using an ancient controller like that. I could find virtually no info on it anywhere.

Where'd you find that info Dan?

Keith Cress
Flamin Systems, Inc.-
 
> Where'd you find that info?

The 3 ring binder on my book shelf in the single loop controller section.

It's dated Dec 1986, the first issue of the UDC, I believe.
I used a number of them.

It was a great controller, first with universal programmable inputs, could handle electric actuator motors with a slidewire card, even had 7 different functions for a dry contact digital input. Didn't have to send a UDC to the local distributor to have the range re-programmed when switching input types or ranges, like I had to when I used Barber Colman 560's.

But the UDC auto/manual button couldn't be disabled and about 80% of calls from customers were because someone had walked by, popped the controller into manual and no one noticed until the PV drifted to wherever a fixed stable output would put it. Honeywell fixed that in the mid '90s.

Most of those blue bezel units have passed on by now, and I wonder what one gets when buying one on eBay . . .

You need a copy? I can fax or scan and email it.

Dan
 
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